The voltage rating and connection of transformers are identified by the five symbols shown below:
Transformer Winding Symbols
Combinations of these five symbols can be used to completely describe the voltage ratings of a transformer. These symbols were adopted by manufacturers because it would otherwise require a lengthy description.
The first voltage of transformers with symbol (/) is the basic coil voltage. A 240/120 transformer has a center tapped 240-Volt coil with a center tap brought out. A 120/240-Volt transformer has two 120-Volt coils which can be connected in series or parallel. When connected in series it can supply 120/240-Volt, 3-wire service.
A 120 x 240 transformer can supply either 120 or 240-Volt service but not 3-wire, 120/240-Volt service.
The number of bushings (insulators) brought out can influence the variety of connections possible. For instance, a 120 x 240-Volt transformer might have only two low-voltage bushings while a 240/120-Volt unit would have three. A 120/240-Volt unit might have three or four. Also, some high-voltage windings are insulated throughout for the full line voltage: these can be connected in either wye or delta. A winding with one high-voltage bushing and one low-voltage bushing must be operated with the low-voltage terminal connected to a solid ground. These single-phase transformers usually are identified by the letter "G" in their description. For instance, a distribution transformer with three low-voltage bushings for use on a three-wire, 240/120-V residential service and a 7,200-V primary winding with only one side insulated for the full line voltage would be described in this way:
Additive Polarity
Terminal Markings For Transformers
